Storm water runoff is considered a form of diffuse or non-point pollution. It is caused by rain flushing pollutants such as particulate matter, nutrients, heavy metals and organic toxins (oil and grease, pesticides, herbicides) into natural bodies of water. Pollution from storm water runoff is aggravated by such activities as land clearing and urbanization, in the latter case due to paving which renders land impermeable to water, acts as a non-adsorbent collection surface for contaminants, and increases runoff rates and volumes.
Several hundred years ago, storm water runoff was not a problem. Rain fell on earth rich in organic matter formed by the accumulation of decayed vegetation. This organic matter absorbed and filtered the water before it made its way into the groundwater, streams and rivers. In addition, the undisturbed soils lost little particulate matter due to erosion, and there was insignificant, if any, amounts of oil and grease, heavy metals or organic toxins to be carried into the receiving wetlands, streams and rivers.
Recognition of the deleterious effects which present-day urban activities and urban storm water runoff have on natural bodies of water and their flora and fauna, has resulted in new regulations for storm water treatment (Novotny, V, "Diffuse (Nonpoint) Pollution--a Political, Institutional, and Fiscal Problem," Journal WPCF 60(8):1404-1413, Ed. 1988a; Novotny V., "Nonpoint Pollution: 1988--Policy, Economy, Management, and Appropriate Technology," American Resources Association, Bethesda, MD, Ed. 1988c; Novotny, V., "Nonpoint Pollution: 1988--Policy, Economy, Management, and Appropriate Technology," American Resources Association, Bethesda, MD Ed. 1988b; Novotny V. et al., "Linking Nonpoint Pollution and Deterioration," Water Environ. & Tech. 1:400-407, 1989; EPA, National Pollutant Discharge Elimination System Permit Application Regulations for Storm Water Discharges; Final Rule, U.S. Environmental Protection Agency, Federal Register 40 C.F.R. Parts 122, 123, and 124, 1990; EPA, Draft Fiscal Year 1989 Nonpoint Source Report to Congress, U.S. Environmental Protection Agency, 1990a; Field, R. et al., "Urban Storm-Induced Discharge Impacts," Water Environ. & Tech. (August):64-67, 1990; Novotny, V., "Urban Diffuse Pollution; Sources and Abatement," Water Environ. & Tech. 3:60-65, 1991; Tarbert, R.E., "The Downpour of Stormwater," Regs. Environ. Prot. (June):27-46, 1991). These regulations require treatment of storm water runoff from urban roadways, industrial sites, parking lots and other facilities where pollution problems due to runoff can occur.
Conventional approaches for treating storm water runoff include wet detention ponds, constructed wetlands, grassy swales (vegetative control) and infiltration basins (Federal Highway Administration, "Retention, Detention, and Overload Flow for Pollutant Removal from Highway Stormwater Runoff," FHWA/RD-87-056, 1988; Horner R.R., "Biofiltration Systems for Storm Runoff Water Quality Control," Washington State Dept. of Ecology, 1988; Roesner, L.A. et al., "Design of Urban Runoff Quality Controls," American Society of Civil Engineers, 1988; King County Department of Public Works, Surface Water Design Manual, King County, WA 1990). Land treatment of wastewaters is widely practiced and, properly designed and operated, can be a highly effective, efficient and environmentally safe method of water pollution control (EPA, Process Design Manual for Land Treatment of Municipal Wastewater, U.S. Environmental Protection Agency, EPA-625/1-77-008, 1977; EPA, Design Manual:Onsite Wastewater Treatment and Disposal Systems, U.S. Environmental Protection Agency, EPA-625/1-80-012, 1980; EPA, Process Design Manual--Land Treatment of Municipal Wastewater, Supplement on Rapid Infiltration and Overland Flow, U.S. Environmental Protection Agency, EPA-625/1-81-013a, 1984b; EPA, Handbook:Septage Treatment and Disposal, U.S. Environmental Protection Agency, EPA-625/6-84-009, 1984a; EPA, Process Design Manual for the Land Application of Municipal Sludge, Environmental Protection Agency, EPA-625/1-83-016, 1985; Kilduff J. E., "Design and Construction of Leaching Systems in Fill Based on Permeability," Journal Environ. Eng. Proceedings of the American Society of Civil Engineers, 115:239, 1989). In addition to nutrient control (Swift, R. S. et al., "Micronutrient Adsorption by Soils and Soil Colloids," In G. H. Bolt et al., Ed. Interactions at the Soil Colloid-Soil Solution Interface, Kluwer Academic Publishers, Boston, 257-292, 1991), land treatment systems are also capable of adsorbing heavy metals (Kirkham, M. B., "Organic Matter and Heavy Metal Uptake," Compost Science Jan.-Feb.: 18-21, 1977; Hutchins et. al., "Fate of Trace Organics During Land Application of Municipal Wastewater," CRC Critical Reviews in Environmental Control 15(4): 355, 1985; Kotuby-Amacher J., et al., "Factors Affecting Trade Metal Mobility in Subsurface Soils," U.S. Environmental Protection Agency, EPA-600/S2-88/036, 1988; Zirschky, J. et. al., "Metals Removal in Overland Flow," Journal WPCF 61:470-475, 1989; Westall, J. C. et. al., "Adsorption of Organic Cations to Soils and Subsurface Materials," U.S. Environmental Protection Agency, EPA-600/S2-90/004, 1990; De Boodt, M. F., "Application of the Sorption Theory to Eliminate Heavy Metals From Waste Waters and Contaminated Soils," In G. H. Bolt et al., Ed. Interactions at the Soil Colloid-Soil Solution Interface, Kluwer Academic Publishers, Boston, 1991; Forstner, U., "Soil Pollution Phenomena--Mobility of Heavy Metals in Contaminated Soil," In G. H. Bolt et al., Ed. Interactions at the Soil Colloid-Soil Solution Interface, Kluwer Academic Publishers, Boston, 1991) and toxic organics (Sheng-Fu, C. et. al., "Aqueous Chemistry and Adsorption of Hexachlorocyclopentadiene by Earth Materials," In D. W. Shultz, Ed. Land Disposal: Hazardous Waste, U.S. Environmental Protection Agency, Cincinnati, Ohio, 29-42, 1981; Fuller, W. H. et al., "Soils in Waste Treatment and Utilization," Vol. I, Land Treatment CRC Press, Inc., Boca Raton, Fla., 1985; Scheunert, I. et al., "Predicting the Movement of Chemicals Between Environmental Compartments (air-water-soil-biota)," In P. Sheehan et al., Ed. Appraisal of Tests to Predict the Environmental Behavior of Chemicals, John Wiley & Sons, Inc., New York, N.Y., 285-332, 1985; Sims, R. C. et. al., "Treatment Potential for 56 EPA Listed Hazardous Chemicals in Soil," U.S. Environmental Protection Agency, EPA-600/S6-88/001, 1988; Chiou, C. T., "Theoretical Considerations of the Partion Uptake of Nonionic Organic Compounds by Soil Organic Matter," Soil Science Society of America, Inc., Madison, Wis., 1989; Pignatello, J. J., "Sorption Dynamics of Organic Compounds in Soils and Sediments," In B. L. Sawhney et al., Ed. Reactions and Movement of Organic Chemicals in Soils, Soil Science Society of America, Inc., Madison, Wis., 1989).
The efficiency of land treatment systems is generally considered to be directly dependent on the soil organic matter content. The average organic matter content of most good agricultural soils lies within the range of 1 to 5 percent (McGraw-Hill Encyclopedia of Science & Technology, Soil, McGraw-Hill Book Co., New York, N.Y. 1987). As the organic matter content of soil increases, the cation exchange capacity (CEC) of the soil also generally increases; i.e., a soil's ability to adsorb waste materials, particularly heavy metals, is highly correlated with the soil organic matter content. However, the limiting factor in land treatment of wastewaters is generally not organic matter content or the CEC; it is primarily based on soil permeability. As the CEC increases, there is usually an increase in clay content of natural soils, and a subsequent decrease in the ability of water to infiltrate and take advantage of the binding sites for purification. For this reason, land treatment of wastewaters requires large surface areas, and can usually be applied only in rural areas or for smaller municipalities. To minimize land area requirements, a soil-like material with a high percentage of stable organic matter, a high CEC, and a high permeability rate is required.
While these methods can be efficient if adequately sized and/or, if soil and groundwater conditions are appropriate, they require relatively large land areas for effective treatment. Land availability and costs are usually not a problem outside of urban areas. However, in urban areas, land costs are high and, in many instances, required land for these conventional storm water treatment technologies is simply not available at any cost. In addition, due to evidence that heavy metals tend to accumulate and concentrate in treatment pond, wetland, or infiltration basin sediments (Nightingale, H. L., "Accumulation of As, Ni, Cu, and P in Retention and Recharge Basins Soils From Urban Runoff," Water Resources Bulletin 23(4):663-671, 1987; Mesuere, K. et al., "Behavior of Runoff-Derived Metals in a Detention Pond System," Water, Air and Soil Pollution 476:125-138, 1989), there is increasing concern over the long-term environmental consequences of such accretion on underlying groundwaters. There also appears to be increasing potential for bio-accumulation of heavy metals and other toxins by the fauna and flora of pond or wetland systems used for storm water treatment.
The type and strength of pollutants found in storm waters will vary greatly and depend on such factors as rainfall intensity, population and traffic density, season, proximity of industrial facilities, and other land use factors. Pollutants found in typical storm water runoff from highway structures in the United States is shown in the following Table 1:
TABLE 1 __________________________________________________________________________ Pollutant Concentrations (mg/l) Group Parameter Average Range Sources Examples __________________________________________________________________________ Particulates TS 1147 145-21640 Tire, Brake & Dust & Dirt, TVS 242 26-1522 Pavement Wear, Stones, Sand, TSS 261 4-1656 Car Exhaust, Gravel, Grain, TVSS 77 1-837 Mud & Dirt Glass, Plastics, Accumulated on Metals, Fine Vehicles Residues Heavy Metals Cd 0.04 0.01-0.40 Lead, Zinc, Iron Cr 0.04 0.01-0.14 Copper, Nickel, Cu 0.103 0.01-0.88 Cadmium, Fe 10.3 0.1-45.0 Mercury Ni 9.92 0.1-49.0 Pb 0.96 0.02-13.1 Zn 0.41 0.01-3.4 Organic Matter BOD5 24 2-133 Vegetation, Dust Vegetation, Litter, TOC 41 5-290 & Dirt, Humus, Animal Droppings, COD 14.7 5-1058 Oils, Fuels Motor Fuels & Oils Oil & Grease 9.47 1-104 Pesticides/ Dieldrin (ppb) 0.005 0.002-0.007 Weed Killers Right-of-Way Herbicides Lindane (ppb) 0.04 0.03-0.05 Maintenance PCB's (ppb) 0.33 0.02-8.89 Nutrients TKN 2.99 0.1-14.0 Nitrogen, Fertilizers NO2 + NO3 1.14 0.01-8.4 Phosphorus PO4 0.79 0.05-3.55 Pathogenic Total C Coliforms Soil, Litter, Bacteria Fecal C Excreta, Bird & (Indicators) Animal Droppings __________________________________________________________________________
In the case of particulates, the average levels for storm waters are higher than those for typical municipal wastewater or sewage in the United States in total solids (TS), total volatile solids (TVS) and total suspended solids (TSS). In addition, the extreme upper range limits found in storm waters can exceed that for wastewaters, e.g., in total volatile suspended solids (TVSS), chemical oxygen demand (COD), and oil and grease. In the case of nutrients, the typical values in storm waters are lower than those for typical wastewater. This clearly indicates the potentially serious adverse impact of storm water runoff on natural receiving water systems.
Storm water also differs from wastewater in other ways. Unlike wastewater, which flows more or less continuously year around, storm water is intermittent, and usually shows seasonal peaks. Pollutant concentrations in storm water, in addition to being highly dependent on localized factors, are also correlated with rainfall interval spacing. In other words, the longer the time span between storms, the greater the pollutant concentration when a rainfall event occurs. This is due to the continual accretion of pollutants on the drained surfaces over time. Thus, potential damage to receiving water ecosystems is greatest after a prolonged dry spell, such as occurs during summer periods over much of the west coast of the United States, when the first storms of the fall season wash particularly concentrated accumulations of toxic materials off roadways and other surfaces. These first flush events occur when receiving streams are at low flow and the dilution of pollutants from storm water is minimal. Thus, these events cause the greatest impacts on receiving water quality. However, this factor is heavily rainfall intensity dependent. Therefore the heaviest pollutant loading at the end of a dry spell may not occur during the first storms, if these storms are not of sufficient strength to fully flush the receiving basin deposition surfaces.
Within a particular storm event, there is also what is known as the "first flush" phenomenon. Generally, the first flush occurs during the first half-hour or so, when the rain is flushing the amassed buildup of pollutants which have accumulated during the interval since the preceding storm, and pollution loadings are highest. Even if the storm lasts several hours or more, contamination levels during the remainder of the event are usually low or even undetectable.
With the foregoing considerations in mind, it is one object of the present invention to provide a storm water runoff treatment system that is capable of accepting and treating pollutant levels which can be greater than those commonly seen in domestic wastewaters and are sometimes more typical of industrial wastewater strengths and compositions.
It is another object of the present invention to provide a storm water runoff treatment system that is capable of treating high volumes of heavily contaminated storm water on an almost instantaneous basis with intervening and often prolonged dry periods.
It is yet another object of the present invention to provide a storm water runoff treatment system that is capable of treating particularly heavy pollutant loadings during the first half-hour or so of a storm water runoff event, followed by influent of greatly diminished concentrations.